Long-haul trucks transport goods over great distances in all parts of the world. In California alone, there are approximately 180,000 transport trucks in operation. Since the operators of long-haul trucks spend many days at a time on the road, the cabins for such trucks typically include a bed, as well as microwaves, air conditioners and heaters, refrigerators, televisions, stereos and other electric appliances that require significant amounts of power. Long-haul trucks equipped with this type of cabin are referred to as sleeper berth vehicles.
In thirty states and the District of Columbia, many different types of vehicles, including sleeper berth vehicles, are not allowed to idle their main engines for a period longer than five minutes, which is why more of these vehicles are installing auxiliary power units (APU's) to run in place of the main engine when the drivers are attempting to sleep or making use of the other convenience features of the vehicles. While these laws are more rigorous in some states, such as California, than other states, emissions standards are becoming increasingly more rigorous nationwide.
According to regulations promulgated by the California Air Resources Board (CARB), all trucks of model year 2007 or higher must have a level three DPF system installed. A level three DPF system is classified as having an 85% or higher reduction in particulate matter or a less than 0.01 g·bhp-hr (grams per brake horsepower per hour) emission level. Additionally, in order to meet level three DPF system requirements, the DPF system must have the exhaust plumbed into the vehicle's exhaust system upstream of the particulate matter after-treatment device. The ultimate goal of CARB's new regulation is to reduce public exposure to diesel particulate matter and other contaminants by limiting the idling of diesel fueled commuter motor vehicles and the emissions created by the idling of such vehicles. With the average truck idling for 2,500 hours per year, one can imagine the amount of particulate matter being put into the air.
With the aforementioned regulations being enforced, it has become necessary for owners and operators of diesel fueled trucks to find an alternative, economical way to operate their trucks without breaking the regulations. Similarly, since some large trucking fleet companies have over 100% turnover from year to year as a result of operator dissatisfaction, fleet companies are always seeking new ways to attract drivers, such as by providing them with a luxurious cab environment that includes many of the comforts and conveniences of home, such as the ability to utilize multiple appliances at one time. Considering that the current average cost to train a new driver is between $3,000 and $5,000, it seems key to the financial success and longevity of a company to do what they can to retain their current employees.
Accordingly, in order to operate multiple appliances, adequately heat/cool the cabin, and maintain happier operators, a number of companies have begun to affix APU's to their trucks to provide climate control and 120 volt power, to cut back on fuel consumption and air pollution, to reduce operating hours on the main vehicle engine, and to improve the overall quality of life of the driver/operator. With the current average price for a gallon of diesel gasoline around $2.37 per gallon and an idling engine consuming 1.2 gallons per hour, it is becoming prohibitively expensive for drivers to idle their engines for long periods of time. With an average idle time of 2,500 hours/year and $2.37 per gallon, idling without an APU costs more than $7,000 per year. A typical APU consumes between 0.2-0.3 gallons per hour, which saves more than $5,000 per year in fuel costs alone. These facts don't consider the new regulations being instituted to reduce air pollution.
While it is known to include a DPF system on certain types of diesel engines, the need to incorporate DPF systems into APU's is a new problem and presents a number of different issues not previously addressed with DPF systems. In particular, many DPF systems on engines are passive regeneration systems because the temperature of the exhaust gases generated by the engines is sufficient to ignite the particulate matter and initiate oxidation in the presence of a wall flow filter, either catalyzed or non-catalyzed. Smaller engines, such as those used for APU's do not typically generate sufficient exhaust temperatures to induce oxidation unassisted. Active regeneration systems, however, are large and expensive, which make their incorporation into an APU impractical. A typical solution to this problem is to equip the APU engines with low exhaust temperatures with large filter units that can collect large amounts of particulate matter before requiring regeneration. While the filters are large, and that can be problematic, the filters can be located remote from the APU on a portion of the truck chassis that affords more space. When regeneration is required, the engine must be serviced to remove the filter and clean out the particulate matter, either using a cleaner or an active regeneration process that will ignite the particulate matter. Naturally, servicing the engine to clean out the filter adds complication to ownership of the engine and DPF system and increases the cost of operating the engine.
In addition to the need to decrease emissions levels, APU's are typically noisy to operate. Even with a muffler installed on the exhaust system, they tend to output a significant level of noise and generate a significant amount of vibration throughout the cabin, especially when the engine is mounted under the cabin or attached to the frame rail of the truck. In many cases, airborne noise, such as that produced by the cylinder exhaust ports of the engine, is propagated in and transmitted through the exhaust pipe wall and radiated as shell noise or transmitted to the vehicle through the engine mounts. As such, mufflers which typically include a resonating chamber that is designed to have opposite sound waves collide and cancel each other out, are utilized to reduce exhaust noise.
Internal combustion engines also typically include an air intake system for receiving air that is later mixed with fuel and combusted in the engine's cylinders. Noise from the engine, however, also typically travels through the air intake system, through the engine, and back into the atmosphere. In certain smaller engines, especially where the exhaust is in close proximity to the air intake, such as in an APU, noise traveling from the engine through the air intake can be a significant noise source. Air intake silencers have typically been used to muffle and reduce the resultant level of noise produced by these types of engines. Additionally, noise reduction is often achieved by the provision of sound dampeners such as baffles within the silencer housing as a result of attenuating the sound waves produced by the entering air. Such silencers are most commonly constructed as a combined air silencer/filter module. However, having a filter placed in the upstream direction of the airflow near the silencer can also generate increased levels of noise.
While mufflers and intake silencers can reduce noise and effectively reduce some level of vibration, the aforementioned noise reduction techniques are not typically focused on vibration reduction which may still exist in the absence of noise being attenuated by the muffler or silencer mechanisms. Furthermore, mufflers, DPF systems and other noise and vibration reduction technologies are often placed within the intake and exhaust systems of an engine, which can have an impact on airflow. Since DPF systems often require back pressure within the exhaust to be tightly regulated to insure a sufficient quantity of particulate matter in the exhaust is removed, it can be difficult to reduce the sound and vibrations generated by an APU when a DPF system is installed because the introduction of the noise and vibration technology may impact the operability of the DPF system.